Update some info on the README.md

* Downloads page should point to julialang.org/downloads, users didn't know about the nightlies somehow!
* Update SuiteSparse instructions to record that after running contrib/repackage*.make, users should still set USE_SYSTEM_SUITESPARSE=1
* OSX Homebrew instructions should point to my tap
* Other minor nitpickings
Merge pull request #3974 from JuliaLang/sf/readmedownload

Update some info on the README.md